1. Field of the Invention
This invention relates to abrasive products having a resinous binder which holds and supports abrasive grains on a backing sheet or in a fibrous sheet.
2. Discussion of the Art
Coated abrasives generally comprise a flexible backing upon which a binder holds and supports a coating of abrasive grains. The backing can be selected from paper, cloth, film, vulcanized fiber, etc. or a combination of one or more of these materials, or treated versions thereof. The abrasive grains can be formed of flint, garnet, aluminum oxide, alumina zirconia, ceramic aluminum oxide, diamond, silicon carbide, etc. Binders are commonly selected from phenolic resins, hide glue, urea-formaldehyde resins, urethane resins, epoxy resins and varnish. Phenolic resins include those of the phenol-aldehyde type.
The coated abrasive may employ a "make" coat of resinous binder material in order to secure the ends of the abrasive grains to the backing as the grains are oriented, and a "size" coat of resinous binder material can be applied over the make coat and abrasive grains in order to firmly bond the abrasive grains to the backing. The resin of the size coat can be the same material as the resin of the make coat or of a different material.
In the manufacture of coated abrasives, the make coat and abrasive grains are first applied to the backing, then the size coat is applied, and finally, the construction is fully cured. Generally, thermally curable binders provide coated abrasives having excellent properties, e.g., heat resistance. Thermally curable binders include phenolic resins, urea-formaldehyde resins, urethane resins, melamine resins, epoxy resins, and alkyd resins. In order to obtain the proper coating viscosities, solvent is commonly added to these resins. When polyester or cellulose backings are used, curing temperature is limited to about 130.degree. C. At this temperature, cure times are long. The long cure time along with solvent removal necessitate the use of festoon curing areas. Disadvantages of festoon curing areas include the formation of defects at the suspension rods, inconsistent cure due to temperature variations in the large festoon ovens, sagging of the binder, wrinkling of very flexible webs, and shifting of abrasive grains. Furthermore, festoon curing areas require large amounts of space and enormous amounts of energy.
Radiation curing processes have been used in an attempt to avoid the disadvantages of festoon ovens. For example, Offenlegungsschrift 1,956,810 discloses the uses of radiation for the curing of unsaturated polyester resins, acid hardenable urea resins, and other synthetic resins especially in mixtures with styrene. U.S. Pat. No. 4,047,903 discloses a radiation curable binder comprising a resin prepared by at least partial reaction of (a) epoxy resins having at least 2 epoxy groups, e.g., from diphenylolpropane and epichlorohydrin, with (b) unsaturated monocarboxylic acids, and (c) optionally polycarboxylic acid anhyride. U.S. Pat. No. 4,547,204 discloses the use of radiation curable acrylated epoxy resins in one adhesive layer of the coated abrasive and the use of a heat curable phenolic or acrylic latex resin in another adhesive layer of the coated abrasive.
Although radiation curable resins solve the problems associated with thermally curable resins, with respect to festoon ovens, the radiation curable resins are generally more expensive than the thermally curable resins. In many abrasive products this increase in cost cannot be tolerated and thermally curable resins are still utilized. Also, radiation curable resins generally do not exhibit the heat resistance necessary for severe coated abrasive applications. In an attempt to solve these problems, U.S. Pat. No. 4,588,419 discloses an adhesive for coated abrasives comprising a mixture of: (a) electron beam radiation curable resin system comprising an oligomer selected from the group consisting of: urethane acrylates or epoxy acrylates, filler, and a diluent and (b) a thermally curable resin selected from the group of phenolic resins, melamine resins, amino resins, alkyd resins, and furan resins. However, the radiation curable resin and the thermally curable resin disclosed in this patent do not co-react or copolymerize. It is desired that the radiation curable resin and the thermally curable resin copolymerize in order to form a tight cross-link network, thereby providing improved thermal properties necessary for severe coated abrasive applications.